712 



SPECIAL SENSES. 



instrument called the perimeter, by which the field is marked on the inner 

 surface of a hemisphere. The field of vision thus delineated is an irregular 



oval, extending from the 

 fixed point, farther to 

 the temporal side than 

 to either the nasal side 

 or above and below. The 

 extent from the fixed 

 point is about 90 on 

 the temporal side, and 

 about 70 to the nasal 

 side and above and be- 

 low. The field for white 

 is larger than for colors,, 

 especially on its nasal 

 side, as is seen in Fig. 

 257. The field is small- 

 est for green, a little 

 larger for red, and is 

 larger still for blue. In- 

 vestigation of the field 

 of indirect vision with 

 the perimeter is very 

 useful in ophthalmolo- 

 gy, but the chief physi- 

 ological interest, as regards the sensibility of the retina, is connected with 

 direct vision. 



FIG. 257. Field of vision of the right eye, as projected by the pa- 

 tient on the inner surface of a hemisphere, the pole of which 

 forms the object of regard semi-diagrammatic (Nettleship, 

 after Landolt). 



T, temporal side ; N, nasal side ; w, boundary for white ; B, bound- 

 ary for blue ; R, boundary for red ; a, boundary for green. 



BINOCULAR VISION. . 



Thus far the mechanism of the eye and its action as an optical instru- 

 ment, in monocular vision only, have been described ; but it is evident that 

 both eyes are habitually used, and that their axes are practically parallel in 

 looking at distant objects and are converged when objects are approached to- 

 the nearest point at which there is distinct vision. In fact an image is 

 formed simultaneously upon the retina of each eye, but it is nevertheless- 

 appreciated as a unit. If the axis of one eye be slightly deviated by pressure 

 upon the globe, so that the images are not formed upon corresponding points 

 in the retina of each eye, vision is more or less indistinct and is double. In 

 strabismus, when this condition is recent, temporary or periodical, as in recent 

 cases of paralysis of the external rectus muscle, when both eyes are normal,, 

 there is double vision. When the strabismus is permanent and has existed 

 for a long time, double vision may not be observed, unless the subject direct 

 the attention strongly to this point. As but one eye is capable of fixing 

 objects accurately, images are formed upon the fovea of this eye only. Images 

 formed upon the retina of the other eye are indistinct, and in many instances- 

 are habitually disregarded ; so that practically the subject uses but one eye,. 



